1. Field of the Invention:
This invention relates to a method for the production of a maleimide. More particularly, it relates to a method for the production of a maleimide by the reaction of maleic anhydride with a primary amine.
2. Description of the Prior Art:
Maleimides are compounds useful as raw materials for synthetic resins, medicines, and agricultural chemicals. Researches after methods for their produciton have long been under way. The most popular method of them all effects the production of maleimides by the dehydration cyclization of maleinamic acids with a dehydrating agent such as acetic anhydride. One version of this method is disclosed in U.S. Pat. No. 2,444,536. This method effects the production of maleimides by causing maleic anhydride to react upon amines thereby forming maleinamic acids and dehydration cyclizing and, at the same time, imidating the maleinamic acids in the presence of acetic anhydride and sodium acetate. This method, however, has the disadvantage that the imidation requires expensive acetic anhydride to be used in at least an equivalent relative to the maleinamic acid and the separation and recovery of the formed maleimide from the imidation reaction solution necessitates use of a large volume of water and, as the result, entails disposal of a large amount of an acetic acid-containing effluent at great expense. Thus, this method may will be called a too expensive method for commercial production of malemidies.
A method which has no use for such a chemical dehydration agent as acetic anhydride is disclosed in British Patent No. 1,041,027 and U.S. Pat. No. 3,431,276. This method effects the production of maleimides by thermally dehydrating and cyclizing maleinamic acids in conjunction with a solvent such as, for example, toluene, xylene, or chlorobenzene having a boiling point exceeding 80.degree. C. and serving as a diluent and an acid catalyst such as sulfur trioxide, sulfuric acid, or ortho-phosphoric acid, and distilling the system thereby causing azeotropic expulsion of the consequently formed water in conjunction with the solvent. As compared with the method which uses acetic anhdyride, this method proves advantageous in that it does not require use of a large amount of such an expensive dehydrating agent as acetic anhydride and further that the formed maleimides are separated and recovered with ease. This method nevertheless has the disadvantage that the yield of the imidation is low as compared with that obtainable by the method using acetic anhdyride. This disadvantage is logically explained by a postulate that compared with the method which effects the imidation by the use of acetic anhydride, the method which effects the imidation by performing thermal dehydration in the specific solvent as described above involves a high reaction temperature and, therefore, tends to induce side reactions and inevitably manages to produce maleimides abounding with impurities and further that since maleimdies are thermally unstable, the maleimides produced at all are degenerated during the course of the reaction. Further, as a commercial process, this method is not ecconomically satisfactory, because it requires to use an expensive acid catalyst in a relatively large amount and, moreover, produces the maleimides in a low yield.
There is another method which, as disclosed in Japanese Patent Laid-Open SHO 53(1978)-68,770 and Japanese Patent Publication SHO 57(1978)-42,043, comprises causing maleic anhydride to react on amines in the presence of an organic solvent thereby forming maleinamic acids and subjecting the maleinamic acids as held in a state not isolated from the reaction system to dehydration and cyclization in the presence of such an aprotic polar solvent as dimethyl formamide or dimethyl sulfoxide and an acid catalyst. By this method, there is offered recognizable improvement in yield as compared with the second method described above. This method, however, has these disadvantages, that the cost of production of maleimides is high because expensive and highly toxic aprotic polar solvent such as dimethyl formamide is used in a large amount, that the solvent such as dimethyl formamide is degenerated by the action of an acid catalyst used in the reaction and, therefore, the solvent is lost greatly, and that since the aprotic polar solvent used in the reaction has a high boiling point, the solvent is removed from the produced malimides with great difficulty.
Japanese Patent Laid-Open SHO 60(1985)-11,465 and Japanese Patent Laid-Open SHO 62(1987)-273,952 disclose a method which produces a maleimide by subjecting maleic anhydride and an amine to condensation in a solvent in the presence of an acid catalyst. To be specific, this method effects the production of a maleimide by preparatorily charging a reactor with the maleic anhydride, the solvent, and the acid catalyst and adding the amine dropwise to the reaction system with the solvent kept refluxed thereby allowing the water formed by the reaction to be expelled as mixed with the solvent from the reaction system. This method, however, has a disadvantage that the maleic anhydride readily reacts with the water formed by the condensation and gives rise to maleic acid and the maleic aicd thermally induces the reaction of rearrangement and entails heavy by-production of fumaric acid.
As a result, the amount of the maleic anhydride to be actually consumed in the reaction is decreased and the yield of the maleimide to be produced is lowered proportionately. Further, the reaction performed under such conditions as described above entails various secondary reactions and forms the product aimed at in a very low yield. Moreover, since the by-produced fumaric acid is insoluble in the acid catalyst and the solvent, it is suffered to accumulate in the boundary between the two liquid layers possibly to an extent of obscuring the interface between the liquid layers. As a result, the separation of the reaction solution and the catalyst to be made after completion of the reaction is attained only with great difficulty. If the separation is carried out with great strain, the catalyst is suffered to mingle with the insoluble fumaric acid at a conspicuous sacrifice of the attributes of the catalyst. The catalyst recovered by the separation, therefore, is cannot be effectively reused in its unmodified form.
As described above, the methods heretofore proposed for the production of a maleimides have numerous problems and cannot be satisfactorily adopted on a commercial scale.
An object of this invention, therefore, is to provide a novel method for the production of a maleimides.
Another object of this invention is to provide a method which is capable of producing a maleimide safely, easily, and inexpensively in a high yield.